Multilayered Regulation of Membrane-Bound ONAC054 Is Essential for Abscisic Acid-Induced Leaf Senescence in Rice

Abstract

In most plants, abscisic acid (ABA) induces premature leaf senescence; however, the mechanisms of ABA signaling during leaf senescence remain largely unknown. Here, we show that the rice (<i>Oryza sativa</i>) NAM/ATAF1/2/CUC2 (NAC) transcription factor ONAC054 plays an important role in ABA-induced leaf senescence. The <i>onac054</i> knockout mutants maintained green leaves, while <i>ONAC054</i>-overexpressing lines showed early leaf yellowing under dark- and ABA-induced senescence conditions. Genome-wide microarray analysis showed that ABA signaling-associated genes, including <i>ABA INSENSITIVE5</i> (<i>OsABI5</i>) and senescence-associated genes, including <i>STAY-GREEN</i> and <i>NON-YELLOW COLORING1</i> (<i>NYC1</i>), were significantly down-regulated in <i>onac054</i> mutants. Chromatin immunoprecipitation and protoplast transient assays showed that ONAC054 directly activates <i>OsABI5</i> and <i>NYC1</i> by binding to the mitochondrial dysfunction motif in their promoters. ONAC054 activity is regulated by proteolytic processing of the C-terminal transmembrane domain (TMD). We found that nuclear import of ONAC054 requires cleavage of the putative C-terminal TMD. Furthermore, the <i>ONAC054</i> transcript (termed <i>ONAC054α</i>) has an alternatively spliced form (<i>ONAC054β</i>), with seven nucleotides inserted between intron 5 and exon 6, truncating ONAC054α protein at a premature stop codon. ONAC054β lacks the TMD and thus localizes to the nucleus. These findings demonstrate that the activity of ONAC054, which is important for ABA-induced leaf senescence in rice, is precisely controlled by multilayered regulatory processes.